1. Field of the Invention
The present invention relates to video signal processing and in particular to a means of enhancing the sharpness of video images.
2. Description of the Related Art
Video peaking techniques have been used to enhance the sharpness of video images. Conventional video peaking techniques typically involve enhancing the “edges” or transition regions within an image by filtering a luminance signal with a high-pass or band-pass filter, scaling the filtered signal with a gain factor, then adding the scaled signal with the original luminance signal to generate an output signal having sharper edges.
Prior art inventions also tend to involve the use of peaking techniques in combination with noise coring techniques which is the process by which a range of signal levels are suppressed (i.e., are not peaked) so as to avoid amplifying the visible noise in the signal.
For instance, prior art document U.S. Pat. No. 6,377,313 discloses an edge enhancement circuit that comprises a high pass filter for filtering an input signal, a noise coring circuit to remove the filtered signal below a defined magnitude, and, a circuit in parallel to the coring circuit to derive a variable peaking strength.
However, prior art solutions are problematic in that, whilst they enhance the edge components of a video signal, they also tend to amplify noise components of the signal which occupy the same frequency range. This results in visible distortion in the displayed video image. Moreover, the noise characteristics of a video signal may vary throughout the signal, and prior art peaking techniques are not sensitive to these fluctuations meaning that optimal peaking and noise coring is difficult to achieve.
Another problem inherent in prior art inventions is that they are unable to scale the filtered signal such that weak edges are enhanced without over-enhancing strong edges.
It is also noted that video transmission signals interleave luminance and chrominance components of a video signal as a “composite signal”. Prior art techniques employ notch filters to decode the chrominance components from the luminance component of the composite signal. However, these filters tend to leave residual chroma in the luminance signals which is susceptible to peaking and appears as moving noise when peaked.
The present invention seeks to alleviate at least some of the problems discussed above.